Second, it is a more continuous record than that taken from rocks on land. First, it is a global record: there is remarkably little variation in isotope ratios in sedimentary specimens taken from different continental locations. As an indicator of shifts in the Earth‘s climate, the isotope record has two advantages. The higher the ratio of oxygen 18 to oxygen 16 in a sedimentary specimen, the more land ice there was when the sediment was laid down.
The degree of enrichment can be determined by analyzing ocean sediments of the period, because these sediments are composed of calcium carbonate shells of marine organisms, shells that were constructed with oxygen atoms drawn from the surrounding ocean. Because heavier isotopes tend to be left behind when water evaporates from the ocean surfaces, the remaining ocean water becomes progressively enriched in oxygen 18. When an ice age begins, the continental ice sheets grow, steadily reducing the amount of water evaporated from the ocean that will eventually return to it. Almost all the oxygen in water is oxygen 16, but a few molecules out of every thousand incorporate the heavier isotope 18.
A recent discovery makes such a determination possible: relative land-ice volume for a given period can be deduced from the ratio of two oxygen isotopes, 16 and 18, found in ocean sediments. To establish such a chronology it is necessary to determine the relative amounts of land ice that existed at various times in the Earth‘s past. For sometime this theory was considered untestable, largely because there was no sufficiently precise chronology of the ice ages with which the orbital variations could be matched. Milankovitch proposed in the early twentieth century that the ice ages were caused by variations in the Earth‘s orbit around the Sun. Certain parts are given in bold to answer some of the questions based on the passage. Click here to view Reading comprehensionĭirections (Q.1-10): Read the following passage carefully and answer the questions given below it.Here we are creating question sample in Reading comprehension, which is BASED ON Bank EXAMS 2018 !!! Welcome to Online English Section with explanation in.
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